The present invention relates generally to an improved method of forming chlorine hydrate, and particularly a method of forming chlorine hydrate for a zinc-chloride secondary energy storage battery system which does not require refrigeration equipment.
The secondary energy storage systems of the type referred to herein (e.g., a zinc-chloride battery system or other suitable metal-halogen battery system) generally are comprised of three basic components, namely an electrode stack section, an electrolyte circulation subsystem, and a store subsystem. The electrode stack section typically includes a plurality of cells connected together electrically in various series and parallel combinations to achieve a desired operating voltage and current at the battery terminals over a charge/discharge battery cycle. Each cell is comprised of a positive and negative electrode which are both in contact with an aqueous zinc-chloride electrolyte. The electrolyte circulation subsystem operates to circulate the zinc-chloride electrolyte from a reservoir through each of the cells in the electrode stack in order to replenish the zinc and chloride electrolyte components as they are oxidized or reduced in the cells during the battery cycle. In a closed, self-contained zinc-chloride battery system, the storage subsystem is used to contain the chlorine gas which is liberated from the cells during the charging of the battery system for subsequent return to the cells during the discharging of the battery system. In such a zinc-chloride battery system, chlorine gas is liberated from the positive electrodes of the cells and stored in the form of chlorine hydrate. Chlorine hydrate is a solid which is formed by the store subsystem in a process analogous to the process of freezing water where chlorine is included in the ice crystal.
With reference to the general operation of a zinc-chloride battery system, an electrolyte pump operates to circulate the aqueous zinc-chloride electrolyte from a reservoir to each of the positive "chlorine" electrodes in the electrode stack. These chlorine electrodes are typically made of porous graphite, and the electrolyte passes through the pores of the chlorine electrodes into a space between the chlorine electrodes and the opposing negative or "zinc" electrodes. The electrolyte then flows up between the opposing electrodes or otherwise out of the cells in the electrode stack and back to the electrolyte reservoir or sump.
During the charging of the zinc-chloride battery system, zinc metal is deposited on the zinc electrode substrates and chlorine gas is liberated or generated at the chlorine electrode. The chlorine gas is collected in a suitable conduit, and then mixed with a chilled liquid to form chlorine hydrate. A gas pump is typically employed to draw the chlorine gas from the electrode stack and mix it with the chilled liquid (i.e., generally either zinc-chloride electrolyte or water). The chlorine hydrate is then deposited in a store container until the battery system is to be discharged.
During the discharging of the zinc-chloride battery system, the chlorine hydrate is decomposed by permitting the temperature to increase, such as by circulating a warm liquid through the store container. The chlorine gas thereby recovered is returned to the electrode stack via the electrolyte circulation subsystem, where it is reduced at the chlorine electrodes. Simultaneously, the zinc metal is dissolved off of the zinc electrode substrates, and power is available at the battery terminals.
Further discussion of the structure and operation of zinc-chloride battery systems may be found in the following commonly assigned patents: Symons U.S. Pat. No. 3,713,888 entitled "Process For Electrical Energy Using Solid Halogen Hydrates"; Symons U.S. Pat. No. 3,809,578 entitled "Process For Forming And Storing Halogen Hydrate In A Battery"; Carr U.S. Pat. No. 4,100,332 entitled "Comb Type Bipolar Electrode Elements And Battery Stack Thereof". Such systems are also described in published reports prepared by the assignee herein, such as "Development of the Zinc-Chloride Battery for Utility Applications," Interim Report EM-1417, May 1980; "Development of the Zinc-Chloride Battery for Utility Applications," Interim Report EM-1051, April 1979, both prepared for the Electric Power Research Institute, Palo Alto, Calif.; "Zinc-Chloride Electric Engine Unit for Four-Passenger Electric Vehicle", SAE Reprint May 1981 from Electric And Hybrid Vehicle Progress, p-91, April 1981; and "Recent Advances In Zinc-Chloride Battery Technology" published in Proceedings of 30Th Power Sources Symposium, June 1982. The specific teachings of the aforementioned cited references are incorporated herein by reference.
Extensive efforts have been taken over many years to develop techniques for forming a halogen hydrate. Indeed, these efforts have given rise to several inventions for which commonly assigned patents have been granted or are currently pending, including: U.S. Pat. Nos. 3,713,888 and 3,809,578 identified above; Bjorkman U.S. Pat. No. 3,783,027 entitled "Apparatus And Method For Making Chlorine Hydrate From High Energy Density Battery Electrolyte And Chlorine"; Behling U.S. Pat. No. 3,793,077 entitled "Battery Including Apparatus For Making Halogen Hydrate"; Bjorkman U.S. Pat. No. 3,814,630 entitled "Filter/Store For Electric Energy Storage Device"; Bjorkman U.S. Pat. No. 3,823,036 entitled "Secondary Battery Comprising Means For Forming Halogen Hydrate Solid Bubble Shells"; Bjorkman U.S. Pat. No. 3,840,650 entitled "Stable Chlorine Hydrate";Symons U.S. Pat. No. 3,907,592 entitled "Halogen Hydrates"; Symons U.S. Pat. No. 3,908,001 entitled "Manufacture of Chlorine Hydrate"; Symons U.S. Pat. No. 3,935,024 entitled "Halogen Hydrates"; Symons U.S. Pat. No. 3,940,283 entitled "Halogen Hydrates"; Carr et al U.S. Pat. No. 4,146,680 entitled "Operational Zinc Chlorine Battery Based On A Water Store"; Behling U.S. Pat. No. 4,115,529 entitled "Halogen Hydrate Formation From Halogen And Finely Divided Aqueous Droplets"; U.S. Pat. No. 4,306,000 entitled "Method Of Cooling Zinc Halogen Batteries" ; Kodali U.S. patent application Ser. No. 310,627 filed Oct. 13, 1981 entitled "Metal Halogen Battery Construction With Improved Technique For Producing Halogen Hydrate"; U.S. patent application Ser. No. 357,742 filed Mar. 12, 1982 entitled "Halogen Hydrate Storage Device For Mobile Zinc-Chloride Battery System"; U.S. patent application Ser. No. 368,892 filed Apr. 16, 1982 entitled "Multiple Stage Multiple Filter Hydrate Store"; and U.S. patent application Ser. No. 358,628 filed Mar. 16, 1982 entitled "Metal Halogen Battery System With Multiple Outlet Nozzle For Hydrate". The specific teachings of these references are incorporated herein by reference. These are now, respectively, U.S. Pat. No. 4,385,099, issued May 24, 1983; U.S. Pat. No. 4,400,446, issued Aug. 23, 1983; U.S. Pat. No. 4,386,140, issued May 31, 1983; and U.S. Pat. No. 4,389,468, issued June 21, 1983.
Generally speaking, it is desirable to concentrate or otherwise form a dense chlorine hydrate in order to reduce the size and/or weight of the hydrate store. This consideration is particularly important when the zinc-chloride battery is used to power an electric vehicle, where the size and weight of the hydrate store will have a significant effect on the operating range of the electric vehicle. Although highly compressed chlorine hydrate has been employed in the past to reduce the size and weight of the hydrate store, one of the principal techniques for increasing the density of the chlorine hydrate is to employ filtration in the hydrate store compartment. Examples of such filtration techniques may be found in the U.S. Pat. No. 3,814,630 and the U.S. patent application Ser. No. 368,892 identified above.
The purpose of the filter in the hydrate store is to separate the compressible particulate chlorine hydrate form the liquid used in the hydrate formation process. As the chloride hydrate enters the store, it is in the form of a dilute slurry, of which approximately three (3) to seven (7) percent is hydrate crystal. However, due to the amount of chlorine gas which is liberated during the charging of the battery, it is not practical to store the chlorine hydrate particles in this dilute slurry. Accordingly, a filter is used to provide a hydrate concentration system for removing as much of the excess liquid as possible. Thus, it will be appreciated that the increase in the density of the hydrate particles in the store will result in a decrease in the size and weight of the battery system.
The practical levels of chlorine hydrate packing by filtration in a zinc-chloride battery system having a capacity on the order 50 kWh appear to be approximately 0.15-0.17 gm Cl.sub.2 /gm of total chlorine and water stored. These levels should be compared with the maximum theoretical chlorine hydrate densities of 0.33 gm Cl.sub.2 /gm where the hydrate is comprised of Cl.sub.2.8H.sub.2 O and 0.4 gm Cl.sub.2 gm where the hydrate is comprised of Cl.sub.2.6H.sub.2 O.
Accordingly, it is a principal object of the present invention to provide a method of forming chlorine hydrate which more closely approaches the maximum theoretical density levels.
It is another object of the present invention to provide a method of forming highly dense chlorine hydrate without requiring a refrigeration system.
It is a further object of the present invention to provide a method of forming highly dense chlorine hydrate for which the heat of formation may be removed directly to the enviroment.
It is an additional object of the present invention to provide a method of forming highly dense chlorine hydrate in situ.
It is another object of the present invention to provide a method of forming highly dense chlorine hydrate such that a relatively smaller hydrate store volume is required.
It is yet another object of the present invention to provide a method of forming highly dense chlorine hydrate as part of a method of charging a zinc-chloride battery system used to power an electric vehicle.
To achieve the foregoing objects, the present invention provides a method of forming chlorine hydrate which generally comprises, combining liquid chlorine with an aqueous liquid at a pressure below 100 psig, and removing the heat of hydrate formation to a substantially ambient temperature environment up to approximately 26.degree. C. This method may also include the steps of mixing the liquid chlorine with the aqueous liquid, and cooling the chlorine hydrate to provide a pressure decrease. However, in accordance with the present invention, no refrigeration of the aqueous liquid is required, as the heat of formation may be released to a substantially ambient temperature environment.
Additional advantages and features of the present invention will become apparent from a reading of the detailed description of the preferred embodiments which makes reference to the following set of drawings in which: